I. Technical Field
The present invention relates to an information recording medium wherein information is recorded, erased, overwritten and/or reproduced optically or electronically, and a method of manufacturing the same.
II. Description of Related Art
In conventional information recording media, a phase-change information recording medium that utilizes a phase-change phenomenon in a recording layer (phase-change material layer) is produced. Among these phase-change information recording media, one in which information is recorded, erased, overwritten and/or reproduced optically by using a laser beam is an optical phase-change information recording medium. In an optical phase-change information recording medium, a change of state of the recording layer is produced as a result of heat generated by irradiation with a laser beam, and the difference in reflectance is detected and is read out as information. Among optical phase-change information recording media, in a overwritable optical phase-change information recording medium in which information can be erased and overwritten, the initial state of the recording layer is generally a crystalline phase, and when information is recorded, the recording layer is melted by irradiating with a high power (recording power) laser beam and then is rapidly cooled, and thus the laser-irradiated portion becomes amorphous. On the contrary, when information is erased, the recording layer is warmed by irradiating with a laser beam which has a lower power than the power used for the recording and then is cooled, and thus the laser-irradiated portion becomes crystalline. Consequently, in a rewritable optical phase-change information recording medium, by irradiating the recording layer with a laser beam for which the power is modulated between a high power level and a low power level, it is possible for new information to be recorded or overwritten while recorded information is being erased. Moreover, among optical phase-change information recording media, for write-once optical phase-change information recording media in which it is possible for information to be recorded one time but not possible for information to be erased or overwritten, the initial state of the recording layer is generally an amorphous phase, and the laser-irradiated portion becomes crystalline as the recording layer warms up and cools while being irradiated with a high power (recording power) laser beam when information is recorded.
Instead of the abovementioned irradiation with a laser beam, there are also electrical phase-change information recording media that record information by causing a state change in the phase-change material of the recording layer by means of Joule heating generated by the application of electrical energy (for example electrical current). In these electrical phase-change information recording media, the phase-change material of the recording layer undergoes a state change between a crystalline phase (low resistance) and an amorphous phase (high resistance) by means of Joule heating generated by the application of electrical current, and the difference in electrical resistance between the crystalline phase and amorphous phase is detected and is reproduced as information.
The commercial 4.7 GB/DVD-RAM is given as an example of an optical phase-change information recording medium. As shown in FIG. 12 for information recording medium 12, the 4.7 GB/DVD-RAM has a 7-layer configuration, where first dielectric layer 2, first interface layer 3, recording layer 4, second interface layer 5, second dielectric layer 6, light absorption correction layer 7, and reflection layer 8 are provided over substrate 1 in this order from the laser incident side.
First dielectric layer 2 and second dielectric layer 6 adjust the optical path and enhance the light absorption efficiency of recording layer 4, so that optical action increases the signal intensity as the change in reflectance between the crystalline phase and the amorphous phase grows larger, and serve a thermal function to insulate the heat-sensitive substrate 1 and dummy substrate 10 and so forth from the heat due to the higher temperature of recording layer 4 during recording. As a dielectric material, for example, conventionally used (ZnS)80(SiO2)20 (mol %) is a superior dielectric material that has transparency and a high refractive index, and is also a good insulator with low thermal conductivity, favorable mechanical characteristics and resistance to humidity. Furthermore, the thicknesses of first dielectric layer 2 and second dielectric layer 6 is determined exactly according to a calculation based on the matrix method, so as to satisfy conditions that increase the change in the amount of reflected light between the crystalline phase and amorphous phase of recording layer 4, and increase the light absorption in recording layer 4.
By using a high crystallization speed material in recording layer 4 that includes Ge—Sn—Sb—Te wherein Sn substitutes for a portion of the Ge in the pseudo-binary phase-change material GeTe—Sb2Te3 that combines the compounds GeTe and Sb2Te3, not only is there efficient overwriting of the initial recording, but superior recording storage stability (the indicator of whether the recorded signal can be recovered after long-term storage) and overwriting storage stability (the indicator of whether the recorded signal can be erased or overwritten after long-term storage) are also realized.
First interface layer 3 and second interface layer 5 function to prevent mass transfer from taking place between first dielectric layer 2 and recording layer 4, and between second dielectric layer 6 and recording layer 4. In this mass transfer phenomenon, when (ZnS)80(SiO2)20 (mol %) is used in first dielectric layer 2 and second dielectric layer 6, S (sulfur) diffuses into the recording layer during the time when recording layer 4 is irradiated with a laser beam for repeated recording and overwriting. When S diffuses into the recording layer, the repeat overwriting capability deteriorates. The use of Ge-containing nitrides in first interface layer 3 and second interface layer 5 favors the avoidance of this deterioration of the repeat overwriting capability (for example, see Japanese published unexamined patent application No. H10-275360 (pp. 2-6, FIG. 2)).
Through the use of technology such as that described above, superior overwriting performance and high reliability were achieved and the 4.7 GB/D VD-RAM was brought to commercialization.
Moreover, various kinds of technology have been studied in order to obtain information recording media with higher capacity. In the example of optical phase-change information recording media, a high density recording technique with a smaller laser beam spot diameter was investigated by using a violet-blue laser with a shorter wavelength than that of the conventional red laser, and by using a thinner substrate on the laser beam-incident side and an objective lens with a larger numerical aperture (NA). When recording is carried out with a smaller spot diameter, since the time irradiating laser beam with the recording layer become relatively short, the recording layer needs to be made up of materials which has high crystallization ability or to be contact with an interface layer which has high crystallization promoting effect so that short time crystallization is available.
In addition, the information capacity increases two-fold by using an optical phase-change information recording medium that is provided with two information layers (referred to below as a bilayer optical phase-change information recording medium), and the technique of carrying out recording and reproducing operations on the two information layers by using an incident laser beam from one side has also been investigated (for example see Japanese published unexamined patent application No. 2000-36130 (pp. 2-11, FIG. 2); and Japanese published unexamined patent application No. 2002-144736 (pp. 2-14, FIG. 3)). In these bilayer optical phase-change information recording media, laser beam that is used will pass through the information layer proximal to the laser beam incident side (referred to as the first information layer) in order to perform recording and reproducing operations on the information layer distal to the laser beam incident side (referred to below as the second information layer), so the first information layer should have an extremely thin film thickness and high transparency. However, as the recording layer becomes thinner, crystal nuclei formed during the crystallization of the recording layer decrease and a distance of atom transfer becomes shorter. Thus, as the recording layer becomes thinner, the harder will be forming the crystal phase (i.e. the crystallization speed becomes low). For the above reasons, in the first information layer where the recording layer is thin, the recording layer needs to be made up of materials which has high crystallization ability or to be contact with an interface layer which has high crystallization promoting effect.
In addition, when the transfer rate of information becomes high by shortening the recording time of the information of the information recording medium, the crystallization time becomes short. Thus, to realize phase-change information recording media which support high transfer rates, the recording layer here also needs to be made up of materials which has high crystallization ability or to be contact with an interface layer which has high crystallization promoting effect.
Heretofore, inventors have introduced materials which have high crystallization ability for the recording layer, and have introduced Ge-containing nitrides in interface layers arranged on both sides of the information layer in substantially the same manner as with the 4.7 GB/DVD-RAM.
However, in a case that the materials which have high crystallization ability for the recording layer is used to improve the crystallization speed of an optical phase-change information recording medium, forming the amorphous state will be hard particularly for a overwritable optical phase-change information recording medium. Thus, the recording layer must be heated to a high temperature to broaden a melting area and then must be rapidly cooled. This requires higher energy (laser power) for the information recording. Then, when the conventional Ge-containing nitrides are used for the interface layers, heat generated in the recording layer destroys the interface layer and the repeat overwriting capability rapidly deteriorates.
Moreover, since Ge-containing nitrides have high thermal conductivity, if a thicker interface layer is constructed, the heat will facilitate the diffusion. As a result, there is a problem with reduced recording sensitivity.
In addition, to improve the repeat overwriting capability, when an interface layer containing Cr and O, not the abovementioned Ge-containing nitrides, is introduced, the repeat overwriting capability improves but the signal storage stability of recording marks deteriorates. Even this interface layer containing Cr and O is used, there is a problem with reduced recording sensitivity.